System for treating a wound with suction and method of detecting a loss of suction

ABSTRACT

A system for applying suction to a wound is provided, including a suction source for providing suction to a conduit operatively associated with the wound to communicate suction to the wound, a vent in fluid communication with the conduit to provide a reference airflow into the conduit when the system is in operation, such that a loss of suction at the wound is indicated by deviation from the reference airflow, and a waste collector canister between the wound and the suction source for collecting liquids aspirated from the wound, wherein the canister includes one or more disks containing gelling agent.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a Continuation Application of utility U.S.application Ser. No. 12/207,241, entitled System for Treating a Woundwith Suction and Method Detecting Loss of Suction, filed Sep. 9, 2008,pending, which is a Divisional Application of utility U.S. applicationSer. No. 11/268,212, entitled System for Treating a Wound with Suctionand Method Detecting Loss of Suction, filed Nov. 7, 2005, now U.S. Pat.No. 7,438,705, which claims priority to provisional U.S. ApplicationSer. No. 60/699,218, entitled Apparatus and System for Suction-AssistedWound Healing, filed Jul. 14, 2005, the complete disclosures of whichare fully incorporated by reference herein.

FIELD OF THE INVENTION

The invention is related to the general field of wound treatment, and tothe more specific field of devices for wound drainage and treatment withsuction.

BACKGROUND OF THE INVENTION

It is well known to treat wounds by applying suction under airtightwound covers. The suction can be used to aspirate wound exudate andother liquids from the wound and/or the wound packing materials, andsuction can be applied as a healing modality for its well knownantiseptic and tissue regeneration effects.

A primary concern in using suction assisted wound therapy is maintainingsuction pressure at the wound. It is increasingly recognized that oncesuction wound dressings are applied, the suction should be maintained atcertain levels. Loss of suction to the wound can result if leaks orblockages occur in the system.

Leaks can occur through breach of the airtight seals surrounding thewound. The earliest devices merely ran a tube under the edge of a woundcover and applied an adhesive or paste around the tube and cover tomaintain an airtight seal. The other end of the tube was connectable toa waste collection reservoir and suction regulator of a hospital suctionsystem, and the pressure selected at the regulator was assumed to be theapproximate pressure communicated to the wound area under the cover. Thetest for leakage was visual observance of the cover contracting and theabsence of whistling noise when negative pressure was applied. See,e.g., M. E. Chariker, et al, “Effective Management of Incisional andCutaneous Fistulae with Closed Suction Wound Drainage”, ContemporarySurgery, vol. 34, June 1989, pages 59-63. This initial check for anairtight seal did not, however, provide any warning if the systemdeveloped a leak or if blockage occurred in the collection circuit afterhours of use.

The adhesive seal around the suction tube or other tubes which run underthe edge of the wound cover in these prior wound dressings is vulnerableto cracking and breaching the airtight seal, creating air leakage intothe cover. Improvements have been made to suction wound therapy devicesto reduce the likelihood of leakage. For example, U.S. application Ser.No. 11/181,128, filed Jul. 14, 2005 and commonly assigned with thisapplication, discloses a tube attachment patch to allow the suction tubeto be terminated outside of the primary cover and thus reduce the riskof breaking the adhesive seal to the skin. While these newer tubeattachment devices provide more seal integrity, there is still thepotential for breach of the airtight seals due to patient movement inbed.

Blockage of suction to the wound can occur for several reasons. A wastecollector for wound exudate is usually positioned somewhere in thesuction line between the wound and the suction source. Waste collectorsincorporate shut-off mechanisms that will shut off suction to the woundwhen the collector is overfilled. Another potential blockage is kinkingor crimping of the suction line itself. Blockage due to debris from thewound and clotting is also a concern. The suction source may also beinadvertently turned off, a line may be inadvertently clamped, or thesuction regulator may be misadjusted after the dressing is applied.Since suction wound dressings are intended to last for long periodswithout changing, usually 24 hours or greater, a leak or blockage coulddevelop unnoticed and not be detected for a duration sufficient tonegate the beneficial healing effect of suction as well as increase therisk of infection. There are currently devices to sense when a wastecollector canister is filled and provide a warning signal to empty it.None of the devices, however, provides a comprehensive and reliablemonitoring of system operation, nor cover the full range of faultpossibilities with leakage or blockage.

In their efforts to improve over these prior art devices, the presentinventors considered several design objectives for a suction wounddressing monitoring system; including the impracticality of locatingexpensive pressure transducers in any of the system components thatshould be single-use disposable items due to contamination by bodyfluids. The designers considered that the same basic system shouldpreferably be effective in detecting and indicating both leaks andsystem blockage, and that the detection of leaks or blockage shouldpreferably be effective regardless of where the leak or blockage occursin the system. They considered that the system should preferably provideclear visual indication of both normal and abnormal operation, and couldusefully provide aural indication and auto-recording of abnormalities.They also considered that the suction monitoring capability should beeasily convertible from use with a stationary suction system, astypically found in hospital care rooms, to use with a portable suctionpump.

These and other advantages and aspects of the invention will becomeapparent upon reading the detailed description and drawings that follow.

BRIEF SUMMARY OF THE INVENTION

The invention provides a system for applying suction to a wound in whicha reference airflow rate is used for monitoring system operation. Thebasic components of a wound treatment system using suction include anairtight wound dressing, a suction conduit having one end operativelyassociated with the wound dressing to communicate suction to the wound,and an opposite end operatively connected with a source of suction,which may be a stationary source such as hospital wall suction or may bea portable pump. The invention provides a reference airflow rate (or“controlled leak”) to the suction source when the system is inoperation, such that deviation from the reference airflow can bemonitored as an indication of a change in operation.

A wide variety of flow rate monitoring instruments can be used todisplay normal operation and detect deviation from the referenceairflow, and several are depicted or described herein. In a stationarysource configuration using a distributed hospital wall suction system,or a limited mobility suction pump using wall receptacle electricalpower, a preferred wound monitoring instrument includes a flow meterthat provides a visual indicator of normal and abnormal airflow rates,and may produce or trigger warning signals indicating abnormalconditions. In a portable battery-powered pump configuration, thepreferred use of a positive displacement pump in which flow rate isproportional to motor rpm allows the airflow rate of the system to beindicated indirectly by the motor speed or pump speed. An algorithm maybe used to correct motor and pump speed to flow for compressibilityeffect. Thus a motor speed indicator or tachometer may be the preferredmonitoring instrument. However, in any of the vacuum sourceconfigurations, other flow rate monitoring instruments could be used,such as, but not limited to, target meters, rotometers, turbine meters,mass flow meters, differential pressure (Dp) cells and hot wireanemometers.

Although the term “airflow” herein for consistency, it should beapparent that it is not limited to the composition of ambient air. It iscommon in medical settings to alter the composition of airflow to awound, such as by increased oxygen or therapeutic aerosols or otherbeneficial medications. The flow of any of these mixed gases or aerosolsuspensions should be considered airflow for purpose of thisdescription.

In a preferred embodiment, a calibrated vent in the suction conduit nearthe wound dressing establishes the reference airflow. Preferably thevent is located in the tube just outside of the wound cover. The ventmay be an aperture in the tube sealed with a porous material havingspecific flow rate characteristics. The reference flow rate may be in apreferred range of 50-300 cc per minute. Higher flows such as 1000cc/min are possible, but practical limitations on the capacity of thesuction source as well as the magnitude of a leak in the dressingsuggest that is preferable to use a flow below 1000 cc per minute.

By locating the vent in the conduit close to the wound, the referenceairflow also serves to purge the conduit of fluids. The purging effectminimizes the likelihood of reflux of contaminated fluid back into thewound, and the risk of infection attendant to such reflux. Since thefluid does not reside in the conduit for an extended time, the purgeeffect also reduces the likelihood of blood clotting in the line.

The system may further include a waste collector, disposed in thesuction line between the wound dressing and the source of suction, forcollecting fluids aspirated from the wound. The preferred collector isan upright transparent plastic canister with fill level markings on theoutside surface. The canister may be prevented from overfilling by ahydrophobic membrane filter at the top of the canister that shuts offthe airflow when the canister is full. The liquids aspirated from thewound may be turned into a gel, for ease of disposal and to preventspilling, by a gelling agent in the disposable canister. To keep thewaste in the canister level, the gelling agent is preferably provided instacked disks. The canister has an inlet connector for the suction tubefrom the wound dressing, and an outlet connector for connecting to aportable pump or to a flow monitoring instrument for use with stationaryor hospital wall suction. The outlet connector is preferably aproprietary (non-standard) connector that will not connect directly tothe standard appliance fitting of a stationary suction system, toprevent anyone from mistakenly connecting directly to the suctionsystem.

The invention also includes the method of using a reference airflow toindicate normal operation of a suction wound therapy system, anddeviation from the reference airflow to indicate leaks or blockage inthe system, using the various apparatus described hereafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded prospective view of a preferred embodiment of asystem for suction wound therapy according to the invention inconjunction with a stationary suction source.

FIG. 2 is a view of a suction tube attachment device showing an air ventin the suction tube to create a controlled reference airflow rate to thesuction source.

FIG. 3 is a cross section view of the wound monitor airflow indicator.

FIG. 4 is a cross-section view of the waste collection canister showingthe shut off membrane and gel agents.

FIG. 5 is an exploded prospective view of a preferred embodiment of asystem for suction wound therapy according to the invention in aportable pump configuration.

FIG. 6 is a schematic diagram of a flow monitor system of a positivedisplacement pump embodiment measuring motor speed as a surrogate ofairflow.

FIG. 7 is a schematic view of a target meter flow indicator alternative.

FIG. 8 is an exploded prospective view of a portion of an alternativeembodiment of a system for suction wound therapy according to theinvention using a therapeutic fluid delivery to the wound.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an embodiment of a system (10) for suction wound therapy.The components include a wound dressing subsystem (12), a suction tube(14), a waste collection canister (16), a flow monitor instrument (18)and a wall suction regulator (20) of a stationary suction source.

The Wound Dressing

The typical wound dressing (12) includes a wound cover (22) and woundpacking material (24), and may include a special wound contact layer(26). It will have a suction tube (14) running under the edge of theprimary cover or through the cover, or will terminate outside of thecover and communicate with a nozzle or slot in the cover. There is awide variety of each of these components. The wound dressing (12) shownin the drawings and described herein uses presently preferredcomponents, but the invention is not intended to be limited to thesecomponents.

The primary wound cover (22) is preferably an airtight wound cover thatis vapor permeable. It is preferred to use a thin film transparentcommercial bandage, such as medical grade Tegaderm™ manufactured by 3M,that is impermeable to liquids but permeable to water vapor and oxygen.The term “airtight” means that the material is sufficiently occlusive toresist airflow through the cover to maintain suction under the coverwhile suction is being applied. There are many other thin-filmcommercial bandages that have similar properties. Highly impermeablematerials could also be used. However, since it is beneficial in woundtherapy to allow water vapor to escape and oxygen to migrate into thewound, semi-permeable covers of film material such as Tegaderm™ arepreferred. In some instances, the primary cover could also be a rigid orsemi-rigid enclosure placed over the wound. The primary wound cover isgenerally sealed to the skin surrounding the wound with an adhesive (notshown) incorporated in the cover, and gap filler paste (not shown) maybe used where needed.

Before the primary cover (22) is applied, the wound is generally packedwith a wound packing (24). Simple gauze or foam pads can be used, orother materials commercially sold as wound packings. The presentlypreferred wound packing, however, is polyester fibers or comparableresilient fibers in a corrugated layer pattern configured to form aresilient compressible structure, as described in U.S. patentapplication Ser. No. 10/981,119, commonly assigned with thisapplication. The wound packing (24) may also be attached to a woundcontact layer (26). The presently preferred wound contact layer (26) hasa special construction that provides a wound contact surface, withdepressions formed into the surface, to work in conjunction with thesuction to encourage more rapid tissue regeneration, as described inU.S. application Ser. No. 10/982,346, commonly assigned with thisapplication. The contact surface is configured such that voids formed bythe depressions remain above the wound surface in the presence ofmoisture, compressive forces and suction, encouraging local tissuedeflection.

The wound dressing includes a suction tube (14) attached to the woundcover. The dressing may have other tubes (28, in FIG. 2) for medicationsupply or wound flushing. The tube(s) may pass under the edge of thecover or through it, but a presently preferred arrangement uses a tubeattachment patch (30) to allow the suction tube to be terminated outsideof the primary cover and reduce the risk of breaking the adhesive sealto the skin, as described in U.S. application Ser. No. 11/181,128,commonly assigned with this application.

As shown in FIG. 2, the distal end of the suction tube (14) in thisembodiment has a vent hole (32) that is sealed with a porous plug (34)that provides a controlled leak, or reference airflow, for the airflowmonitoring devices described hereafter. It is preferable to locate thevent (32, 34) as close to the wound cover as possible. Having the tubeattachment outside of the primary cover allows the use of thiscontrolled leak reference airflow without drying out of the wound, asmight occur if the vent were under the cover near the wound. While, itis feasible to place the controlled leak in the actual wound space,consideration would need to be given to avoid excessive drying of thewound tissue due to the introduced airflow. In addition, it is possiblewith the type of tube attachment patch (30) shown in FIG. 2 anddescribed in U.S. application Ser. No. 11/181,128 to provide the vent asan aperture in the patch (30).

The reference airflow rate from the calibrated vent is used formonitoring system operation. The controlled leak rate should be low tomaintain a proper suction at the wound site and a moist healingenvironment. Preferably the reference airflow is in the range of 50-300cubic centimeters per minute. This flow is low enough to minimize dryingand does not significantly alter the suction applied at the wound.Additionally, the flow is low enough to minimize the use of batterypower where a portable pump is used as the suction source. A flow ofthis magnitude is readily obtained by providing a vent hole (32) in thesuction tube that is sealed sterile with a porous plug (34), asdescribed above. A suitable seal material is a porous plasticmanufactured by Porex from sintered high density polyethylene. Suchporous materials are routinely employed to admit air into medical fluidlines at a suitable flow while maintaining an efficient sterilefiltration barrier against microbes.

The wound therapist applies the wound dressing (12) to the patient andattaches the system to suction. The therapist remedies leaks in thedressing by smoothing out creases or wrinkles in the wound cover andaddressing gaps due to folds in the patient's anatomy. It has at timesbeen useful to address specific areas of difficult anatomy by using agap filling paste, as is common in ostomy care, such as coloplast. Theairflow monitoring instruments described hereafter facilitate theinitial dressing set up process by providing active feedback to thetherapist on the integrity of the seal around the wound cover and tubes.

The Waste Collector

Since a primary function of suction therapy is to drain liquids from thewound, the devices normally include a waste collector (16) forcollecting fluids aspirated from the wound. As shown in FIG. 3, thecollector (16) is preferably an upright plastic canister (36) that mayhave fill level markings on the outside of the canister. The canister ispreferably a disposable, single-use device. The canister has a lid (38)incorporating a fitting (40) passing through the lid for the attachmentof an inlet tube (42). The inlet tube (42) may be an end of the suctiontube (14) leading from the wound, or be a short section of tube thatterminates with a connector fitting (44) for mating with a matchingconnector (46) on the end of the suction tube (14).

Airflow and any blood or other fluid aspirated from the wound passesthrough the fitting (40) in the lid and into the canister (36). Theliquid is retained in the canister, while the air rises and is drawnthrough a hydrophobic membrane (50) sealed to the inside of the lid. Themembrane (50) acts as a bacteria filter and as a shut off mechanism toprevent the canister from overflowing and to prevent contamination fromflowing out of the container toward the suction source.

The lid defines an air channel from the outlet port (52) to anotherconnector (54) for connecting the canister to a portable pump or to atube running to a stationary wall suction system. The second connector(54), however, is preferably a proprietary (non-standard) connector thatwill not connect directly to the standard appliance fitting of astationary suction system. This is to prevent anyone from mistakenlyconnecting directly to the suction system without the flow monitoringinstrument and pressure release features described hereafter, with theattendant risk of applying too great a suction to the wound. The matingreceptacle in the portable pump configuration described hereafter isconfigured to receive the proprietary connector. The connector (54)includes a sealing element such as an O-ring seal. This arrangementprovides that a fresh O-ring is preferably used with each disposablecanister, as contrasted with other pump connectors where the sealingelement is in the pump and will ultimately wear out and cause the systemto fail because of leakage.

The air passage through the canister thus forms a part of a suctionconduit between the wound and the suction source. In the systempresently preferred and described herein, the canister (36) is preventedfrom overfilling by the hydrophobic membrane (50). If the canister (36)is allowed to fill high enough for the contents to contact the membrane,the membrane occludes and blocks airflow from passing through thecanister. This blockage prevents suctioning more waste from the wound,and as described hereafter, the blockage of reference airflow isdetected as an indication of an abnormal operating condition.

Preferably the liquid waste sucked from the wound is turned into a gelfor ease of disposal and to prevent spilling or splashing. Although thegelling agent could be provided in porous bags, it is preferred to usedisks (56) of laminated fiber sheets containing a gelling agent, such asprovided by Gelok International. The disks (56) are cut to fit inside ofthe jar and stacked on top of each other. The stacked disks cause theliquid to turn to a level surface gel in the canister.

The system may also include a filter/drier unit (21) disposed betweenthe waste collection canister and the wound flow monitor (19). Thefilter element provides additional protection against contaminatedparticles entering the suction source, and the drier removes moisturefrom the air that may effect the calibration of the flow monitoringinstrument.

Use of Controlled Reference Airflow to Indicate Normal Operation

While prior art suction dressing systems either do not monitor systemoperation at all, or do so less than optimally by trying to sensepressures directly, the system described herein uses a referenceairflow, as may be provided by the air vent (32) described above, toindicate when the system is in normal operation, and also to detectdeviation from the reference airflow as an indication of abnormaloperation. Higher airflow than the reference flow indicates leakage,while lower airflow indicates blockage.

The range of flows suitable for reference flow are bound by thepermeability of the wound cover on the lower end and suction limits onthe upper end. When the wound cover is a semi permeable material, therewill be a natural low level background airflow from air moleculespassing through the cover. This permeation airflow could serve as thereference flow if the flow monitoring instrument were very sensitive.However, the permeation flow is usually too low and too erratic to be agood reference. The area of the cover and the possibility of it beingobstructed by drapes and fluid from the wound can cause the permeationflow to be erratic. Thus, the vent provides a larger and more stableflow rate that masks variation in the permeation flow and provides agood reference. For example, Tegaderm™ bandage will allow a diffusion ofwater on the order of 800 grams/square meter/day, which correlates to aflow rate of about 0.5 cc/min over a square meter. Typical wound coverarea would be less than a tenth of a square meter, so in approximateterms, the semi permeable material contributes 0.05 cc/min, or less than0.1% of a reference flow of 50 cc/min. Thus, any variation in thebackground permeation flow is masked by the larger reference flow.

The indicator sub-system is embodied in two devices as depicted anddescribed below; a wound monitor flow instrument for use with astationary suction source and a portable pump with a flow sensingsystem. Other types of flow indicating devices, such as target meters orhot wire anemometers could also be used.

The Flow Monitor

In the wound suction system of FIG. 1, the flow rate monitor is a floatmeter (19) that includes a suction conduit (58) between the wastecollection canister (16) and a stationary suction source (20). One endof the conduit includes a mating connector (60) to the proprietaryconnector (54) of the canister. The flow monitor (18) is preferablylocated at the other end of the conduit (58) and attaches directly to anappliance connection fitting (64) associated with an adjustable suctionregulator (20) as commonly found on stationary suction systems.

The flow monitor (19) is designed to provide a visual indication of theflow rate through the system. Once the flow rate stabilizes at thecontrolled reference airflow following initial application, themaintenance of this visual indication of reference flow is an indicationof normal operation free of leaks or blockage, and hence an acceptablelevel of suction at the wound cover. Deviation to a higher airflowindicates leakage, and drop to lower or no airflow indicates blockage inthe collection circuit.

FIG. 4 is a cross section of the float meter (19). A standard connector(63) on the top end of the indicator attaches to a short conduit (62)connectable to the appliance fitting (64) of the adjustable suctionregulator (20). At the bottom end of the flow indicator a standardconnector (59) is attached to the suction conduit (58). It is typical touse suction in the range of 25-200 mm Hg in wound therapy. The flowindicator incorporates a flow restrictor (66) having an orifice (67)that is calibrated to restrict suction airflow to be less than 20 litersper minute when a controlled level of suction of 100 mm Hg is applied bythe suction regulator (20).

As shown in FIG. 4, the flow indicator has progressive sections (68, 70,72, 74) with different inside diameters, having the narrowest section(68) at the bottom and progressively larger diameters in the threeadditional sections (70, 72, 74) progressing to the top. The outsidebarrel (76) of the clear tube has graduated markings (not shown) toindicate airflow rates. A float (78) serves as an indicator of flowthrough the system. When there is no airflow, the float will rest in thelowest section (68). This area is color coded red to indicate a blockagecondition, such as a full canister or any occlusion in the fluid path.When the flow into the meter is low, preferably between about 50 cubiccentimeters per minute to 300 cubic centimeters per minute, theclearance between the float and the bottom section of the indicator issuch that the flow around the float will cause the float to rise intothe next section (70). Thus, normal reference airflow will cause thefloat to rise a discernable amount into the next section. This area ofnormal operation is color coded green, indicating an acceptablecondition.

The mass of the float works in cooperation with the clearance betweenthe outside of the float and the inside of the tube. The float respondsto airflow rate and fluid density and will rise to a level where thedynamic forces are in equilibrium. Typical flow indicators for otherapplications have continuously increasing inner diameters. Slightchanges in flow rate cause the float to chatter up and down around aheight indicative of the flow rate. The use of discrete sections ofuniform diameters, but increasing in progression, as the four sections(68, 70, 72, 74) in this indicator reduces the chatter and causes thefloat to move upward in stages. These stages are selected for discreteflows that provide key reference information regarding the suctionapplied to the wound.

If flow through the system increases beyond an acceptable level due to aleak in the wound dressing, the clearance around the float in the normalposition may no longer be adequate to allow the float to remain in thisposition, and the increased flow will lift the float to a higher sectionin the indicator where the internal diameter is stepped up. Graduatedlabeling (not shown) on the outside of the indicator provides a visualindication of a moderate leak condition. Additional stepped up diametersections are provided to indicate higher leak conditions. The stepped updiameters require a significant change in flow before the float willjump from one position to a higher one. This eliminates the jumpinessthat would be encountered with a continuous taper inner diameter ascommon in flow meters for other applications. The stepped diametersections reduce user interpretation, thus enhancing ease of use andsafety.

The wound monitor instrument also incorporates a safety valve (80) thatlimits maximum suction to guard against the accidental application ofexcessively high levels of suction. It is typical to use suction in therange of 25-200 mm Hg for wound drainage and therapy. Consequently, thesuction limiting feature may be set to limit the suction toapproximately 200 mm Hg. A suction pressure relief chamber (84) isformed between the first flow restrictor (66) and a second flowrestrictor (82) having an orifice (83) the same size as the firstrestrictor. A resilient valve mechanism (86) in the relief chamberserves as a release against the application of high levels of suction.The valve mechanism is spring loaded to a predetermined suction setting,such as 200 mm Hg. If the suction pressure in the relief chamber exceedsthe predetermined setting, the valve mechanism will open and allow ventair to be drawn from a relief port (69) into the metering orifice toprevent excess suction from being applied to the patient.

Protrusions (75) may be provided on the top underside of the interior ofthe float meter adjacent the first flow restriction orifice to preventthe float from completely blocking the orifice entry to the pressurerelief chamber, thus allowing air passage even if a major leak occurs.This allows some suction to communicate through the system even at highairflow rates. Alternatively, the protrusions could be located on thetop surface of the float.

The wound flow monitor instrument is also protected from sources ofpotential contamination in the form of particulate matter, aerosols andhumidity by an air filter/dryer (21) located in the conduit between theinstrument and the canister. The filter/dryer unit includes a particlefilter for removing airborne dust as may be encountered when the woundmonitor is disconnected from the collection canister for periodicchangeover. It is also anticipated that a desiccant or other means ofhumidity control can be placed in the filter/dryer unit as required.

While this system has been described in the context of a hospital wallsuction system, it can also be used with suction pumps. Some medicalbuildings may use electrical power pumps to provide suction for woundtherapy. These can be moved, since they only restriction is the locationof electrical outlets and the length of the power cord, but they are nottruly portable. A system using portable battery powered pump isdescribed below.

Portable Pump

As shown in FIG. 5, a portable pump embodiment (100) uses the same wounddressing subassembly and waste collection canister as described above,but employs a portable suction pump unit (102) instead of a stationarysuction source. The portable pump embodiment is configured to providecontrolled levels of suction to the wound, and has a flow rate monitorto indicate abnormal operation conditions in a manner analogous to thatdescribed for the stationary embodiment. The pump operates with lowvoltage DC and has an onboard power source. The pump will run on wallpower with a suitable power converter, which may be combined with thebattery charger to charge the battery during use. The pump is turned ONusing the control dial (112) on the pump housing.

When the portable pump is a positive displacement pump, it is possibleto monitor pump rpm as a surrogate for airflow rate. As shown in theschematic diagram of FIG. 6, the portable pump embodiment includes apositive displacement air pump (103), a variable speed D.C. motor (104),a tachometer (105), a compressor control (106) and a faultdetection/alarm system (108). The positive displacement pump (103) ispreferably a diaphragm style pump with inlet and outlet check valves.The diaphragm pump includes a crankshaft style input that isoperationally connected to the shaft of the variable speed motor. Otherpositive displacement pumps may be used, such as peristaltic, piston,syringe or rocking piston pumps.

The variable speed motor (104) is preferably a DC brushless motor,however other styles of DC motors are acceptable, as well as AC motorsthat can be equipped with variable outputs. It is preferable to have amotor that is equipped with some form of a tachometer (105) to indicateshaft rpm as the input to the pump. For example, Hall sensors arecommonly employed on DC brushless motors, as well as encoders, toindicate rpm. It is also possible to sense back EMF in brush versions ofDC motors to determine motor shaft rotation speed. In an AC pump, anoscillating bar with a magnet mounted to an end is excited by an ACcoil, thus driving a diaphragm. Pump input in this case is a count ofthe oscillations delivered by said coil. Each of these and other formsof pump or motor speed sensors is included in the generic termtachometer, and can be used as the flow rate monitoring instrument witha positive displacement pump.

The compressor control (106) is typically a pressure transducer and acontrol circuit that permits the user to select a pressure setting, andthe circuit will function to maintain the suction pressure at the inputof the pump at a level consistent with the selection. Such controlcircuits are commonly employed in industrial applications as well as inhospital central suction systems. Compressor control circuits areadjusted to maintain suction or pressure levels in a specified range.This range is selected to provide a reasonably stable suction levelwhile minimizing on-off cycling of the motor. In this portable pumpembodiment, a range of approximately 10 mm Hg is preferred for patientcomfort and to minimize noise. For example, at a setting of 100 mm Hg,the compressor control will nominally maintain a suction pressure at theinlet of the pump in the range of 95 to 105 mm Hg.

The fault detection system (108) is a circuit that is adapted to monitorthe flow of air through the collection circuit coupled with anannunciator to provide audible and visual indication of status ofoperation. This fault detection system is a separate circuit that isindependent of the compressor control. In the preferred embodiment,motor or pump revolutions are counted as an indication of air volumethat has passed through the positive displacement pump. A discriminationalgorithm takes the count of motor revolutions and determines a flowrate. This flow rate is compared to the known reference flow rate todetermine fault conditions. A flow rate that is less than the referenceflow of 50-300 cc/minute is indicated as abnormal operation on theannunciator as an occlusion in the circuit, most commonly a fullcanister. A flow rate that is comparable to the reference flow rateproduces an annunciator indication of normal operation. Flow rates thatare higher than the reference flow rate will produce abnormalannunciator indications that the circuit is leaking to alert thecaregiver to address the seal of the wound cover. If the caregiver isable to correct the leaking dressing the annunciator will indicatenormal operation. Thus, the fault detection system of the portable pumpis analogous to that of the wound monitor flow instrument.

It is sometimes desirable, to benefit the healing process, to providesuction in an intermittent fashion. Prior processes involve allowing thesystem to vent to atmospheric pressure for a period of time to allow forreperfusion of tissue. Allowing the system to completely vent toatmosphere results in the complete loss of suction on the dressingsystem and any benefit that suction could play in maintaining an airtight seal around the wound. The present invention solves this problemby varying the suction between two distinct levels; a first level and asecond level of 20-25 mm Hg. The first and higher level is employed toapply the beneficial effects of suction. The second and reduced level isbelow capillary bed pressure, so reperfusion of tissues will occur inthis lower stage, while maintaining at least some level of suction toaid in keeping the dressing in place and to maintain the referenceairflow to prevent reflux and clotting in the lines. With this type ofintermittent pump operation, the reference airflow should be selectedsuch that it is maintained even at the reduced level, or else the alarmsassociated with low airflow should be disarmed during reduced suctionintervals as well as the ramp-up and ramp-down intervals.

The annunciator may be in the form of a bar meter (114) color coded andgraduated to provide a visual indication that the flow through the pumpis within a range required to maintain the reference airflow when thesystem is in normal operation; and conversely indicate that the pump isrunning faster than required to maintain the reference airflow,indicating air leakage in the system, or is running slower than requiredto maintain the reference airflow, indicating air blockage in thesystem. The pump may also have warning or caution lights (116, 118, 120)indicating pump operating parameters, or other parameters such as lowbattery charge. The pump may also provide an audible indication ofabnormal operation, such as a beeper tone.

The pump is connected to the proprietary fitting (54) of the wastecanister, and may have a conforming docking station (55) to support thecanister. The prescribed pressure setting is selected by a dial (112),which preferably allows set-point choices of 25, 50, 75, 100, 125, 150,175, or 200 mm Hg. Electronic controls provide the ability to enablenumerous combinations of fault indications, both visual and audible.

Target Meter Alternative

There are other devices for determining the flow through the system thatcould be employed as leak detection devices in a wound dressing systemoperating with a reference airflow. These devices measure airflowdirectly, and thus can be used with non positive displacement pumps andwall vacuum sources as well as with positive displacement pumps. Asshown in FIG. 7, a target meter (120) is a reliable device for flowdetection that is insensitive to changes in pressure. The target meter(120) employs an inlet orifice (122) and a target (124) typically placedin close proximity to the orifice. The target is attached to a shaft(126) and pivots on points (128) or an armature. A light torsion spring(not shown) may be used to keep the target in close proximity to theorifice. The extension of the shaft is placed outside of the flow path.As airflow enters the orifice, the target is forced away from theorifice. A proximity sensor, such as a reed switch (130) is so placed asto detect the positions of a magnet (132) on the end of the shaftextension and thus the presence or absence of flow in the circuit aswell as measuring flow rate. A target meter could be placed between thewaste collection canister and the pump or at the outlet of the pump, orbetween the waste collection canister and a stationary suction source toprovide analogous flow rate information as previously described.

Flow Measurement Alternatives

Other flow detection devices that could be used alone or in conjunctionwith a suction source include a hot wire anemometer that determines flowas a function of the cooling of a wire in a flowing air stream, arotameter with an optical detector that senses the position of a float,a turbine meter that rotates when flow is applied. These flow detectionsystems are generally insensitive to changes in pressure. A hot wireanemometer could be placed between the waste collection canister and thepump or at the inlet of the pump, or between the waste collector and ora stationary suction source. These alternative flow sensors allow theuse of other styles of pumps that are not positive displacement, such asvane pumps and scroll pumps. Almost any flow instrument could be used,including, for example, differential pressure (DP) cells that provide anindirect measure of flow rate.

When any fault condition is determined within the system, the pump willcontinue to run, delivering a sub-optimal level of suction, until thecondition is corrected or the user powers the system off, since asub-optimum level of suction applied to the wound is preferable to nosuction at all.

The pump may also have an onboard compliance monitor. This alerts thecaregiver to deviations from the normal application of suction to thewound. The compliance monitor may compute the number of hours thatclinically useful suction has been applied to the wound as determined bythe relative flow through the system. Medical experience has shown thatsuction should be applied for at least 22 of any 24 continuous hours foroptimum efficiency. This readout can be presented a number of ways toshow the number or percentage of compliant hours of suction therapy. Apercentage can be reduced to a dimensionless parameter indicatingacceptable or unacceptable compliance.

Therapeutic Fluid Delivery and Reference Flow

In the embodiments described above, the reference flow for discerningbetween normal and abnormal operation has been ambient air supplied tothe suction conduit through a vent. A reference airflow could beprovided, however, in the form of a therapeutic mixture, such as oxygenenriched air, nitric oxide, heated humidified air or an aerosolcontaining medication droplets or particles. Such a configuration isshown in FIG. 8. A source of therapeutic mixture (115), such as anoxygen regulator or a nebulizer, is connected to a conduit (117) leadingto the wound. The mixture is conveyed through the conduit under orthrough the wound cover (122) and released into the wound packing (124)adjacent the wound contact material (126). Part of this mixture will beabsorbed, but enough could be returned through the suction conduit (114)to provide a reference airflow when the flow has settled intoequilibrium.

While the airflow monitor could be associated with a portable pump or infront of the appliance fitting of a suction regulator, as describedabove, an airflow monitor (119) could be provided between thetherapeutic mixture source (115) and the wound cover (122). Since thepart of the conduit beneath the cover would be contaminated in use, adetachable disposable tube (121) would be used for the section ofconduit from the monitor to the wound packing. Thus, the invention canprovide for the maintenance of suction therapy on a wound as well as theability to monitor the application and removal of therapeutic substancesto the wound.

Although the invention has been described and illustrated with respectto several embodiments described above, it should be understood theinvention may be embodied in other forms without departing from itsscope or attributes. Hence, the scope of the invention should be derivedfrom the claims that follow, recognizing that the claim language mayliterally encompass structure not specifically described.

1. A system for applying suction to a wound, comprising: a suctionsource for providing suction to a conduit operatively associated withthe wound to communicate suction to the wound; a vent in fluidcommunication with the conduit to provide a reference airflow into theconduit when the system is in operation, such that a loss of suction atthe wound is indicated by deviation from the reference airflow; and awaste collector canister between the wound and the suction source forcollecting liquids aspirated from the wound, wherein the canisterincludes one or more disks containing gelling agent.
 2. A system forapplying suction to a wound, comprising: a portable positivedisplacement pump for providing suction to a conduit operativelyassociated with the wound to communicate suction to the wound, the pumpbeing capable of operation at variable speed; a vent in fluidcommunication with the conduit to provide a reference airflow into theconduit when the system is in operation, such that a loss of suction atthe wound is indicated by deviation from the reference airflow; a flowrate monitor to monitor airflow through the conduit and to indicatereference airflow or deviation above or below the reference airflow; anda pressure transducer for detecting pressure at the pump, wherein thepump speed is controlled to maintain the pressure sensed by thetransducer based on a pressure setting; wherein the airflow isdetermined by counting pump revolutions over a period of time.
 3. Asystem as in claim 2, wherein the suction source is a portable pump.